Sometimes, for diagnostic purposes and/or for radiation treatment planning, the target region of the patient may be imaged using a CT system. For example, CT imaging may have interventional and surgical applications, e.g., checking if inserted heart valve is working properly. As another example, cone beam CT (CBCT) imaging may be performed to check patient positioning, setup patient, etc. For the case in which the target region moves in a periodic motion (e.g., due to breathing and/or cardiac motion), the CT system may be used to determine volumetric images of the target when the target is at different breathing states and/or cardiac states, so that the volumetric images may be played back as respective video streams for breathing motion and cardiac motion. To this end, projection images of a target patient at various breathing and cardiac states are obtained.
After the imaging session, the projection images at various stages of a breathing or cardiac cycle are sorted into different sets according to the recorded states of the patient when the corresponding projection images are acquired. For example, the projection images may be sorted according to the phase of the physiological cycle at which they are generated, so that projection images are sorted into different phase bins. After the projection images are sorted, the projection images in each of the phase bin are then used to reconstruct a volumetric image for that phase bins. Alternatively, projection images may be sorted according to the breathing amplitudes of the physiological cycle, so that the projection images are sorted into different amplitude bins. As used in this specification, the term “bin” may refer to phase bin or amplitude bin. Further details on reconstructing volumetric images based on breathing states are disclosed in U.S. patent application Ser. No. 13/436,908, now issued as U.S. Pat. No. 9,047,701, which is herein incorporated by reference in its entirety for all purposes.
However, when taking both breathing motion and cardiac motion in account, improved approaches are required to appropriately sort the various images into respective combined phase bins that correspond to both breathing motion and cardiac motion, and to accurately reconstruct volumetric images for these combined phase bins.